it's not necessary to create a user - if you've heard anything about lindows, then they're an example of a linux company that only uses the root user. the problem this creates doesn't really have anything to do with stability, but security. one of the reasons there are hardly any linux viruses is because it's almost impossible for a virus to do any real damage if it's not the root user. same thing with any other security problem.
in truth, using root all the time on a desktop will not cause any real short-term problems, since root can do anything a user can. the problem is mainly related to servers. it is, however, very good practice to just create a user with regular user privledges and just use that all the time. if you ever want to configure or play around with your system more, or if you make an unfortunate mistake, for whatever reason, you'll be thankful.